This laboratory has been engaged in the isolation and purification of histocompatibility antigens of the rabbit. There are approximately 12 serologically defined haplotypes in the domestic rabbit and our study has concentrated on the RLA 11 haplotype. The major source of antigen has been a tumor cell line derived from the Bar Harbor B strain rabbit. Antigen isolation was carried out by detergent solubilization of cells labelled with tritiated amino acids. Partial amino acids sequence analysis was carried out on the histocompatibility antigen isolated and it revealed a high degree of homology, especially to histocompatibility antigens isolated from humans. Beta-2 microglobulin, a low molecular weight protein that is associated with histocompatibility antigens, was subjected to total amino acid sequence analysis. Although this protein is homologous to analogs from other species, some significant differences were observed in its structure.